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Title: SU-F-T-527: A Novel Dynamic Multileaf Collimator Leaf-Sequencing Algorithm in Radiation Therapy

Abstract

Purpose: A novel leaf-sequencing algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimator (MLC). The efficiency of this dynamic MLC leaf-sequencing method was evaluated using external beam treatment plans delivered by intensity modulated radiation therapy technique. Methods: To qualify and validate this algorithm, integral test for the beam segment of MLC generated by the CORVUS treatment planning system was performed with clinical intensity map experiments. The treatment plans were optimized and the fluence maps for all photon beams were determined. This algorithm started with the algebraic expression for the area under the beam profile. The coefficients in the expression can be transformed into the specifications for the leaf-setting sequence. The leaf optimization procedure was then applied and analyzed for clinical relevant intensity profiles in cancer treatment. Results: The macrophysical effect of this method can be described by volumetric plan evaluation tools such as dose-volume histograms (DVHs). The DVH results are in good agreement compared to those from the CORVUS treatment planning system. Conclusion: We developed a dynamic MLC method to examine the stability of leaf speed including effects of acceleration and deceleration of leaf motion in order to make sure the stability ofmore » leaf speed did not affect the intensity profile generated. It was found that the mechanical requirements were better satisfied using this method. The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.« less

Authors:
;  [1];  [2]
  1. Hefei University of Technology, Hefei, Anhui (China)
  2. Princess Margaret Cancer Centre, Toronto, ON (Canada)
Publication Date:
OSTI Identifier:
22649111
Resource Type:
Journal Article
Resource Relation:
Journal Name: Medical Physics; Journal Volume: 43; Journal Issue: 6; Other Information: (c) 2016 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
60 APPLIED LIFE SCIENCES; 61 RADIATION PROTECTION AND DOSIMETRY; ALGORITHMS; BEAM PROFILES; COLLIMATORS; PHOTON BEAMS; PLANNING; RADIOTHERAPY

Citation Formats

Jing, J, Lin, H, and Chow, J. SU-F-T-527: A Novel Dynamic Multileaf Collimator Leaf-Sequencing Algorithm in Radiation Therapy. United States: N. p., 2016. Web. doi:10.1118/1.4956712.
Jing, J, Lin, H, & Chow, J. SU-F-T-527: A Novel Dynamic Multileaf Collimator Leaf-Sequencing Algorithm in Radiation Therapy. United States. doi:10.1118/1.4956712.
Jing, J, Lin, H, and Chow, J. 2016. "SU-F-T-527: A Novel Dynamic Multileaf Collimator Leaf-Sequencing Algorithm in Radiation Therapy". United States. doi:10.1118/1.4956712.
@article{osti_22649111,
title = {SU-F-T-527: A Novel Dynamic Multileaf Collimator Leaf-Sequencing Algorithm in Radiation Therapy},
author = {Jing, J and Lin, H and Chow, J},
abstractNote = {Purpose: A novel leaf-sequencing algorithm is developed for generating arbitrary beam intensity profiles in discrete levels using dynamic multileaf collimator (MLC). The efficiency of this dynamic MLC leaf-sequencing method was evaluated using external beam treatment plans delivered by intensity modulated radiation therapy technique. Methods: To qualify and validate this algorithm, integral test for the beam segment of MLC generated by the CORVUS treatment planning system was performed with clinical intensity map experiments. The treatment plans were optimized and the fluence maps for all photon beams were determined. This algorithm started with the algebraic expression for the area under the beam profile. The coefficients in the expression can be transformed into the specifications for the leaf-setting sequence. The leaf optimization procedure was then applied and analyzed for clinical relevant intensity profiles in cancer treatment. Results: The macrophysical effect of this method can be described by volumetric plan evaluation tools such as dose-volume histograms (DVHs). The DVH results are in good agreement compared to those from the CORVUS treatment planning system. Conclusion: We developed a dynamic MLC method to examine the stability of leaf speed including effects of acceleration and deceleration of leaf motion in order to make sure the stability of leaf speed did not affect the intensity profile generated. It was found that the mechanical requirements were better satisfied using this method. The Project is sponsored by the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry.},
doi = {10.1118/1.4956712},
journal = {Medical Physics},
number = 6,
volume = 43,
place = {United States},
year = 2016,
month = 6
}
  • Quality assurance (QA) procedures for intensity modulation radiation therapy (IMRT) usually involve an ion chamber measurement in a phantom using the beam configuration of the actual treatment plan. In our QA procedures it was observed that the degree of agreement between the measurement and the calculation could vary from plan to plan, from linac to linac, as well as over time, with a discrepancy up to 8%. In this paper we examine one aspect of the process which can contribute to such poor reproducibility, namely, the leaf end position accuracy. A series of measurements was designed to irradiate an ionmore » chamber using small beam segments where one multileaf collimator (MLC) edge covers half of the chamber. It was shown that the reproducibility varied up to 13%, which provides a possible explanation for the observed discrepancies above. A useful tool was also developed to measure ionization signals from individual segments of an IMRT sequence. In addition, an understanding of the leaf end position variations offers some insight into the overall quality of an IMRT dose distribution.« less
  • Due to practical limitations such as inter- and intraleaf transmission, nondivergent leaf end design, and leaf scatter, multileaf collimators (MLCs) are unable to accurately produce the ideal fluence patterns generated by inverse planning systems. Consequently, low dose regions receive substantially more radiation than they would with an ideal MLC that could generate the desired fluence pattern. Previous work by others has found that the discrepancy between desired and actual fluence patterns produced by an MLC increases rapidly with increasing complexity of the desired fluence map. In addition to the complexity of individual fluence maps, other parameters can contribute to themore » overall complexity of a treatment plan, most notably the number of beams. In this work, we investigate the effect of beam number on critical structure sparing for dynamic MLC delivered intensity modulated radiation therapy. Six cases from each of two challenging clinical sites, previously irradiated head and neck and paraspinal metastasis, were planned with the goal of minimizing the spinal cord dose. Plans were developed for five to 27 beams. All plans were renormalized such that the target volume receiving the prescription dose was the same for all plans of each site. For each case, we calculated the spinal cord D{sub 0.5cm{sup 3}} (the dose such that 0.5 cm{sup 3} of normal tissue receives greater than or equal to D{sub 0.5cm{sup 3}}), normal tissue D{sub 1cm{sup 3}}, the normal tissue mean dose, and the standard deviation of dose in the planning target volume (PTV). For the head and neck cases, the mean increase in spinal cord D{sub 0.5cm{sup 3}} between seven and 27 beam plans was 10% of the prescription dose, whereas for the paraspinal case, the increase was 2.6%. For the head and neck cases, the mean decrease in normal tissue D{sub 1cm{sup 3}} between seven and 11 beam plans was 2.6% and was constant for more than 11 beams. For the paraspinal cases, the mean decrease in normal tissue D{sub 1cm{sup 3}} between seven and 27 beam plans was 3.7%. The mean normal tissue dose was approximately independent of the number of beams for both sites. For the head and neck cases, the PTV standard deviation was independent of the number of beams, while for the paraspinal cases it decreased by an average of 1.8% from seven to 27 beams. Calculations for seven and 27 beams in which the MLC transmission was varied from 0% to 2% demonstrated that the increase in spinal cord D{sub 0.5cm{sup 3}} with increasing number of beams is largely due to MLC transmission, which is not included in the optimization. Increasing the number of beams increased the critical structure dose, although decreasing beam number results in increasing normal tissue D{sub 1cm{sup 3}} and target dose heterogeneity. The optimal tradeoff is dependent on the clinical situation, but seems to be seven to nine beams. Beam geometry optimization may reduce the number of beams required to provide adequate target coverage, thus limiting critical structure dose.« less
  • Purpose: To develop a deliverable four-dimensional (4D) intensity-modulated radiation therapy (IMRT) planning method for dynamic multileaf collimator (MLC) tumor tracking delivery. Methods and Materials: The deliverable 4D IMRT planning method involves aligning MLC leaf motion parallel to the major axis of target motion and translating MLC leaf positions by the difference in the target centroid position between respiratory phases of the 4D CT scan. This method ignores nonlinear respiratory motion and deformation. A three-dimensional (3D) optimal method whereby an IMRT plan on each respiratory phase of the 4D CT scan was independently optimized was used for comparison. For 12 lungmore » cancer patient 4D CT scans, individual phase plans and deformable dose-summed 4D plans using the two methods were created and compared. Results: For each of the individual phase plans, the deliverable method yielded similar isodose distributions and dose-volume histograms. The deliverable and 3D optimal methods yielded statistically equivalent dose-volume metrics for both individual phase plans and 4D plans (p > 0.05 for all metrics compared). The deliverable method was affected by 4D CT artifacts in one case. Both methods were affected by high vector field variations from deformable registration. Conclusions: The deliverable method yielded similar dose distributions for each of the individual phase plans and statistically equivalent dosimetric values compared with the 3D optimal method, indicating that the deliverable method is dosimetrically robust to the variations of fractional time spent in respiratory phases on a given 4D CT scan. Nonlinear target motion and deformation did not cause significant dose discrepancies.« less
  • Purpose: Continuous tumor position measurement coupled with a tumor tracking system would result in a highly accurate radiation therapy system. Previous internal position monitoring systems have been limited by fluoroscopic radiation dose and low delivery efficiency. We aimed to incorporate a continuous, electromagnetic, three-dimensional position tracking system (Calypso 4D Localization System) with a dynamic multileaf collimator (DMLC)-based dose delivery system. Methods and Materials: A research version of the Calypso System provided real-time position of three Beacon transponders. These real-time three-dimensional positions were sent to research MLC controller with a motion-tracking algorithm that changed the planned leaf sequence. Electromagnetic transponders weremore » embedded in a solid water film phantom that moved with patient lung trajectories while being irradiated with two different plans: a step-and-shoot intensity-modulated radiation therapy (S-IMRT) field and a dynamic IMRT (D-IMRT) field. Dosimetric results were recorded under three conditions: no intervention, DMLC tracking, and a spatial gating system. Results: Dosimetric accuracy was comparable for gating and DMLC tracking. Failure rates for gating/DMLC tracking are as follows: {+-}3 cGy 10.9/ 7.5% for S-IMRT, 3.3/7.2% for D-IMRT; gamma (3mm/3%) 0.2/1.2% for S-IMRT, 0.2/0.2% for D-IMRT. DMLC tracking proved to be as efficient as standard delivery, with a two- to fivefold efficiency increase over gating. Conclusions: Real-time target position information was successfully integrated into a DMLC effector system to modify dose delivery. Experimental results show both comparable dosimetric accuracy as well as improved efficiency compared with spatial gating.« less
  • Intensity-modulated radiation therapy (IMRT) is a powerful technique in planning the delivery of dose. The most common IMRT delivery requires the use of moving multileaf collimators (MLCs) to deliver the requested fluence pattern. A dynamic delivery IMRT field file will contain several control points that are defined MLC shapes at a marked fraction of the delivered monitor units. The size of this file and the fidelity of the deliverable fluence are proportional to the number of control points defined. This study investigates the effect of reducing the number of control points has on the resultant dose distribution quality in complexmore » IMRT in efforts to reduce transfer times, loading times, check sum times and file storage. Analysis was performed with 6 head and neck patients on an Eclipse version 8.5 treatment planning system (Varian, Palo Alto, CA). To ensure the quality of all treatments, Eclipse defines a minimum of 64 and a maximum of 320 control points per subfield (Eclipse Algorithms Reference guide). All 6 patients' plans were calculated with fixed 64, 166, and 320 control points using the sliding window technique. In addition, each plan was calculated in variable mode (Normal mode) in which the planning system determined the required number of control points. Each of the 4 plans for each patient was renormalized to provide the same mean planning target volume (PTV) 70 dose. Dose values for critical and target structures were examined for each patient. When examining the minimum, maximum, and mean doses to all target structures, it was noted that the greatest reduction in target dose coverage caused by reduced number of control points was 0.5%, which occurred for the minimum dose to the PTV56 structure in one plan.' Dose analysis for critical structures showed no clinically significant increase in dose when compared with the 320 control point plan.« less